The present invention relates to granulated fertilizer compositions as well as to a method for preparing same, and more especially, to granulated fertilizer compositions based upon alkali-containing calcined phosphates, i.e. mineral phosphates calcined with alkali, containing tetrapotassium pyrophosphate as an adjuvant resulting in improved properties of the composition.
Many proposals have already been made for the granulation of finely divided, ground fertilizer phosphates, either alone or in combination with other fertilizer components. In connection with the so-called build-up granulation method carried out on granulizing trays, in rotating drums or the like, water or steam is typically introduced since the dry substances do not permit the formation of compact granules without moisture. In order to fully comply with the various requirements which are placed upon the granular compositions in practice, additional substances are frequently added to the compositions as adjuvants.
Numerous substances are described in the literature as granulating adjuvants and/or binding agents for fertilizer compositions (see, for example, "Uebersicht In Aufbereitungstechnik", 1971, page 677). In addition to water soluble materials, such as sodium carbonate, sulfite waste liquor, sugar, molasses, calcium chloride (German Auslegeschrift No. 1,153,036), magnesium sulfate, magnesium chloride (German Patent No. 1,258,878), and urea, swellable viscose solutions or products capable of forming suspensions, such as starch, water glass, cellulose ether, alginate, synthetic resins, gum arabic, dextrine, and vinyl resins (German Offenlegungsschrift No. 2,023,502) are mentioned above all. Also, mineral acids are said to be effective (German Offenlegungsschriftten No. 1,767,781 and No. 1,592,754), and in addition, there are mentioned compounds insoluble or difficultly soluble in water, such as clay, bentonite, gypsum, calcium hydroxide, industrial dusts (German Offenlegunsschrift No. 2,060,927), bitumen, tar, wax, or substances capable of reacting with one another, such as magnesium oxide MgO and magnesium chloride MgCl.sub.2, or water glass and calcium chloride.
It is not possible to put forth a completely general rule in connection with the addition of adjuvants, since the effectiveness is most often strongly dependent upon the nature and/or composition of the fertilizer material. In addition the pH value and other factors, which shall not be gone into any further at this point, also play a role.
The relationship between adjuvant and fertilizer is especially complicated when water-insoluble or difficulty soluble fertilizer become involved. To this category belong, for example, the alkali-containing products prepared in a calcining process from naturally occurring calcium phosphates. They are generally considerably more difficultly and imperfectly granulizable than the water soluble fertilizer phosphates. While some of the above-mentioned proposals and/or methods in the case of calcined phosphates (also with mixtures thereof with other nutritive carriers) lead to fine-grained, apparently good granules, nevertheless, one or more of the mechanical properties of the granules are more or less unsatisfactory, or the granulation yield is poor. Moreover, combinations of different methods do not bring any significant improvement.